It is well known that the spark timing in a spark ignited internal combustion engine affects the operation of the engine. As the spark timing for a cylinder is advanced before the top dead center position of the piston, fuel consumption decreases and the output torque increases to a predetermined value. However, if the spark timing is advanced too far, knocking or anomalous combustion will occur. Severe knocking will seriously damage the engine, for example, by melting the piston or by over stressing the mechanical components of the engine. Knock also causes a loss in fuel economy and torque. Many factors affect the spark timing at which knocking begins in an engine. The octane rating of the fuel, the engine speed and combustion chamber design, the cooling capacity of the cylinder walls, the spark plug heat range and the air to fuel ratio are some of the factors which affect the onset of knock. Also, combustion deposits on the walls of the combustion chamber will lead to an octane requirement increase (ORI) which will vary between the different combustion chambers in an engine.
In a typical multicylinder engine, there is a variation in combustion between the different cylinders. Variations in the temperature of the combustion chamber walls and in deposits on the combustion chamber walls may cause one combustion chamber to begin to knock prior to the other combustion chambers as the spark timing is advanced. Or, one combustion chamber may begin to knock prior to the others due to variations in the air to fuel ratio at the different combustion chambers. The combustion process within a single combustion chamber also may vary from cycle to cycle. For a given engine, ignition timing is set for the worst combustion chamber in order to prevent any occurrence of knock. However, knock may still occur in one or more combustion chambers with changes in fuel quality, unless the engine is retuned. If the onset of knock can be detected automatically for each combustion chamber in an engine and the spark timing can be controlled individually for each combustion chamber in response to incipient knock detection, the engine can be operated at a higher efficiency without the occurrence of knock.
Several different methods have been used for detecting knock in an operating internal combustion engine. The most common method senses engine vibrations. Knock causes an engine to vibrate in a specific frequency range. When vibrations are sensed within this range, some prior art systems retard the spark timing to eliminate the knock. When a vibration type sensor is used, the timing is retarded equally for all combustion chambers since vibration sensors cannot easily discriminate between individual combustion events. Systems of this type have several disadvantages. The electrical output of the vibration sensing knock detector must be filtered to remove background noise, thereby increasing the response time and reducing the sensitivity of such detectors. Also, vibration sensing knock detectors are responsive only after the engine is well into the knock region since the knock must be sufficient to appreciably vibrate the engine. Vibration sensing knock detectors are not responsive to incipient knock and are very sensitive to their location on the engine. Holographic methods are often required to identify the best detector location.
It also is known in the prior art that when knock occurs in an internal combustion engine, there is a pressure increase and oscillations in the combustion chamber and the combustion gases are ionized for a short time. By applying a voltage across an electrode gap within the combustion chamber and looking at the ionization current, the pressure increase within the combustion chamber can be detected to indicate the occurrence of knock. Such a system is shown, for example, in U.S. Pat. Nos. 2,543,141 to Vichnievsky 4,232,545 to Dobler et al. 4,262,524 to Russo et al. and 4,444,172 to Sellmaier et al. disclose systems in which ionization is detected at the gap of a conventional spark plug after the spark plug is fired to initiate combustion. Each of these patents disclose a sytem for detecting knock in a combustion chamber, but none disclose details of a control for modifying spark timing to prevent knock in response to the sensed condition. U.S. Pat. No. 4,308,519 to Garcea et al. uses two spaced probes in the combustion chamber to detect ionization caused by knock and discloses decreasing or slowing down the rate of increase in spark advance in response to the output from the probes.
The prior art also teaches that the exhaust gas from the engine is ionized by a post combustion process. When the air to fuel mixture is increasingly leaned out, the combustion process is displaced to a greater extent into the domain of the expansion stroke of the piston until a well-defined post combustion process takes place within the exhaust system of the engine. The magnitude of this post combustive reaction can be detected by means of an ion current sensor located downstream of the exhaust valve within the engine exhaust system, as shown in U.S. Pat. No. 4,372,270 to Latsch et al., wherein a conventional spark plug is mounted in the engine exhaust manifold as an ionization sensor. A voltage is applied to the spark plug and the resulting current when ionized gas is present at the sensor is measured and integrated to detect the air to fuel ratio. This patent suggests using the ion detector signal for adjusting the air to fuel ratio or for adjusting spark advance, although no details are provided for implementing such an adjustment. Nor is there a teaching of individually adjusting spark advance for each combustion chamber since the ionization sensors for each combustion chamber are connected in parallel.